Traditional radiography employs a silver halide photosensitive film in combination with an intensifying screen or screens, which are typically composed of a phosphor layer on a support unit, to capture a radiographic image. The resulting black and white image can then be used for medical diagnosis.
Radiographs can also be made using storage phosphor materials in place of prompt emitting intensifying screen phosphors. No film is needed. The storage phosphors store a latent image in the form of trapped charge that is subsequently read out, typically with a scanning laser beam.
Modulated x-radiation can also be imaged using a solid state detector. In these detectors, a converter is coupled with a pixellated array of detection elements. The converter absorbs the x-ray radiation on an image-wise basis and either emits lower energy radiation, such as visible light, or produces electron-hole pairs. The pixellated detection element array receives the emitted radiation or electron-hole pairs and produces a signal modulated in correspondence with the x-ray radiation intensity. A typical detection element array suitable for use with a converter that emits lower energy radiation is an array of photosensitive elements. Examples of these photosensitive elements are photodiodes coupled to individual transistors of a thin-film transistor (TFT) array, or a TFT array itself, or phototransistors. In this type of detection element array, once the lower energy radiation has been converted into electron-hole pairs in the photosensitive elements, the charge is collected and then read out. This can be done in a row-by-row fashion, in a colunm-by-column fashion using the TFT at each pixel position as a switch or pixel by pixel. A similar TFT array can be utilized with a converter that outputs electron-hole pairs, except that the photosensitive elements are unnecessary. Other types of detection element array can also be used instead of a TFT array, for example, charge coupled devices and charge transport devices.
The detection element array is deposited or otherwise attached to a substrate that provides physical support, for the detection element array and converter. This substrate material is, in many cases, a glass. These substrates currently contain some heavy element addenda to alter the interactions of the substrate with the semiconductor material that is deposited upon it. Physics of Semiconductor Devices, S. M. Sze, John Wiley & Sons, 1981, pages 392-393 discusses some of the reasons these addenda are incorporated in the substrate.
One of the key parameters for radiographs is the modulation transfer function (abbreviated herein as "MTF"). This is a measure of the spatial resolution of the particular radiographic system. It is desirable to provide x-ray imaging detectors having the highest MTF achievable.
Known solid state x-ray detectors having a pixellated conversion-detection unit disposed on a support unit. The conversion-detection unit includes a converter or converter layer in combination with a detection array adjoining the converter. In such x-ray detectors, the x-rays from the x-ray beam source (hereafter also referred to the "primary" x-rays or "primary" beam) are not completely absorbed by the converter. Some x-rays are absorbed by the detection array and some pass through the detection array to encounter the support unit. Some atoms, upon absorbing the x-rays emit lower energy "fluorescent" x-rays (hereafter also referred to as "secondary x-rays"). These secondary x-rays are generated by the absorption of x-rays above the K or L absorption edges of the elements. An electron in the K or L shell is ejected by the absorption of the incident x-ray or other high energy particle. When electrons cascade down to fill this vacant lower energy state, secondary x-rays can be produced at energies characteristic for a particular element. Once an incident high energy particle has been absorbed, the efficiency of production and energy of secondary x-rays are dependent upon the atomic number of the element. The secondary x-rays can be emitted in any direction, but those emitted back to the converter can degrade the image to varying degrees depending on the overall design and construction of the detector.
From the foregoing discussion, it should be apparent that there is a need within the art for an x-ray detector that will effectively control the emission of secondary x-rays.